Gas Bubbles

> I'm still trying to grasp what I few others have been trying to explain
> about "partial pressures" and such, but it seems to me that I've frequently
> heard plant gurus state that carbon dioxide is somehow "driven out" by
> aeration, implying that somehow the additional dissolved gases replace the
> CO2.  Is this true, or is the CO2 removed by some other mechanism?  And of
> course if it is true, why couldn't the equilibrium shift the other way under
> certain conditions?

Aeration is a simpler situation to explain.  You can basically ignore the
effect of the oxygen and other gasses entirely; it's not the other gases
that "drive out" CO2, but the difference in concentration of CO2 itself
between the tank and the surrounding atmosphere.  CO2 will move from one
to the other, always from higher concentration to lower concentration. 
The idea is that when you aearate the water, it increases the water-air
surface area, allowing the CO2 to more quickly reach equilibrium with the
CO2 level in the atmosphere.

If you are trying to supply CO2 to the water in a higher concentration
than you would get in equilibrium with the atmosphere (for instance,
injecting it from a bottle, or having your fish exhale it into the water),
then aerating the water causes the CO2 to diffuse out more quickly than if
the water is still. 

You could look at CO2 in the fish tank as similar to the heat level in an
open refrigerator.  If you open the door a lot or blow a fan into it, the
temperature equalizes with the outside quickly.  If you flail your arms
slow enough or keep the door just cracked open, the refrigerator keeps up
with you and it's at least a LITTLE colder than the surrounding area.

An interesting side point is that if you do not inject CO2 externally, and
your plants quickly consume what is available in the tank (noticable as pH
rise during the day), it might be an ADVANTAGE to aerate your water so
that CO2 diffuses IN quicker from the atmosphere.

The analogy of this would be a badly ventilated auditorium full of sweaty
people on a cool summer day.  Opening doors to the outside and installing
some fans would be advantageous! 

   - Erik
Erik D. Olson					         amazingly, at home
eriko at wrq_com